Anal sphincters

The anal canal is about 3 – 4cm long in women and sometimes slightly longer in men. The internal and external sphincters from 2 concentric rings run along the length of the anal canal.

The internal anal sphincter (IAS) is made of smooth muscle and we do not have voluntary control of this muscle. It works automatically to keep the anus closed until we are ready to have a bowel movement.

The external anal sphincter (EAS) is made of striated muscle (the same as the pelvic floor muscles); we do have voluntary control over the EAS – allowing us to hold on if we are aware of wind or diarrhoea.